SIP (Session Initiation Protocol) is an IETF standard protocol for
interconnecting telephone devices over TCP/IP networks. GNU SIP Witch
is a call server which implements the SIP protocol standard while
supporting generic phone system features like call forwarding, hunt
groups and call distribution, call coverage and ring groups, holding
and call transfer, as well as offering SIP specific capabilities such
as presence and messaging. GNU SIP Witch will support use of secure
telephone extensions and enables communication privacy through the use
of peer-to-peer audio and video sessions directly between connected
endpoints. GNU SIP Witch also supports placing and receiving calls
directly with remote users over the public Internet without requiring
the use of mediating VOIP "service providers" in what is commonly
called SIP "Business-to-Business" (b2b) calling.

GNU SIP Witch can be used together with and to interconnect common
free software and IETF standard compliant voice and video capable SIP
based desktop softphone applications such as Ekiga, Twinkle, Linphone,
and OpenWango, as well as most standard compliant SIP telephone
devices. When used together with a standard compliant SIP media
application server such as GNU Bayonne, GNU SIP Witch will be able to
offer users and remote callers access voice messaging and media
application services. In the future GNU SIP Witch will offer STUN
services and optional packet forwarding to facilitate interconnection
when behind NAT's and firewalls in IPV4 networks. GNU SIP Witch also
supports IPV6.

GNU SIP Witch is not a SIP proxy, a multi-protocol telephone server,
or a IP-PBX, and does not try to address the same things like
asterisk, yate, or GNU bayonne2, all of which make use of direct media
processing where media connections and streaming are at least
initially established between each endpoint and the IP-PBX server
itself. Direct media processing adds additional latency, and by
introducing a central point where all media streams can be processed,
compromises both the privacy of calls and security of encrypted
telephone sessions.

Instead, GNU SIP Witch focuses on doing just one thing as a pure SIP
call server, and will try to do that one thing very well. My goal is
to focus on achieving a network scalable telephone architecture built
around the SIP protocol that can be deeply embedded, which can support
secure calling nodes, that can integrate well with other SIP
based/standards compliant components, and, by not engaging in media
processing, that is not license encumbered by the use of "mandated"
patent encumbered and proprietary telephony media codecs.

GNU SIP Witch requires the GNU oSIP library and eXosip extension
library, and the (GNU) uCommon/GNU Common C++ 2.0 core library. GNU
SIP Witch is offered under the terms of the GNU General Public License
Version 3 or later, and has no dependencies on non-free software. GNU
SIP Witch can be compiled and used on any GNU/Linux distribution
supporting GCC 2.95 or later, has been tested with OpenSolaris,
Free/Open/NetBSD, OS/X, QNX Nutrino, and can be built for use on
Microsoft Windows targets using Debian GNU/Linux hosted MinGW32
toolchains. GNU SIP Witch is a free software package that is part of
the GNU Project and the GNU Telecom Subsystem.